blockgame/mods/bg_player/run.lua

local utils = load_file("util_misc")

local walk_speed = 1
local walk_time = 4
local speedup_time = 6
local run_factor = 2.5
local slowdown_factor = 12
local jump_height = 1
local jump_factor = 1.4

local speed_data = {}

local function handle_speed (player, delta_time)
	utils.handle_player(player, function (player, p_data)
		speed_data[player] = speed_data[player] or {}
		local data = speed_data[player]

		data.duration = (data.duration or 0) + delta_time

		local control = p_data.control
		local moving = control.up or control.down or control.left or control.right
		if not moving then
			data.duration = data.duration - delta_time * (slowdown_factor + 1)
		end
		if data.duration < 0 then data.duration = 0 end
		if data.duration > walk_time + speedup_time then data.duration = walk_time + speedup_time end

		if data.duration < walk_time then
			p_data.physics.speed = walk_speed
			p_data.physics.jump = jump_height
		else
			local run_time = data.duration - walk_time
			local t = run_time / speedup_time

			local factor = (run_factor - 1) * t + 1
			if factor > run_factor then factor = run_factor end

			p_data.physics.speed = walk_speed * factor
			p_data.physics.jump = jump_height + (jump_height * jump_factor - 1) * t
		end
	end)
end

blockgame.register_globalstep(function(delta_time)
	for _, player in pairs(minetest.get_connected_players()) do
		handle_speed(player, delta_time)
	end
end)